1. Field of the Invention
The present invention concerns a method to adjust at least one shim current in a gradient coil of a magnetic resonance apparatus and an associated RF center frequency for a radio-frequency antenna of the magnetic resonance apparatus during an interleaved, multislice MR measurement of a moving examination subject, as well as a magnetic resonance apparatus and an electronically readable data storage medium to implement such a method.
2. Description of the Prior Art
The magnetic resonance (MR) is a known technique with which images of the inside of an examination subject can be generated. The examination subject is positioned in a strong, static, homogenous basic magnetic field (also called a B0 field) with field strengths of 0.2 Tesla to seven Tesla or more in a magnetic resonance data acquisition unit (scanner), such that nuclear spins in examination subject orient along the basic magnetic field. Radio-frequency excitation pulses (RF pulses) are radiated into the examination subject to produce nuclear magnetic resonances, and the resulting, emitted magnetic resonance signals are detected, and MR images are reconstructed based on these signals. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. For example, an associated MR image can be reconstructed from the k-space matrix populated with values by means of a multidimensional Fourier transformation of those values.
Magnetic resonance apparatuses are known that have a support device (for example an examination bed or table) that can be automatically driven by means of an actuator device into and out of a patient receptacle of the magnetic resonance apparatus in which the basic magnetic field is present for the acquisition of magnetic resonance images. Since the patient receptacle frequently has quite a small diameter, the patient is placed on the patient bed outside of said patient receptacle, after which the patient bed can be driven automatically into the patient receptacle by the actuator device.
MR measurements with the examination bed driven continuously through the magnet of the MR system are known. These serve to extend the region that can be depicted—known as the “field of view”—in the direction of the bed displacement (FOVz) and to simultaneously limit the measurement region within the magnet.
The technique competing with the continuous bed feed (also called table feed) is the acquisition of an extended FOVz in multiple stations with a stationary bed. After all data of a station are acquired, the patient is driven with the examination bed to the next station in order to conduct the measurement again there. The measurement (acquisition of data) is interrupted during the movement of the examination bed.
A detailed overview of known MR techniques with continuous table feed is found in the article by Peter Börnert et. al.: “Principles of Whole-Body Continuously-Moving-Table MRI”, Journal of Magnetic Resonance Imaging 28:1-12 (2008).
The most important MR techniques with continuous table feed can be roughly subdivided into 2D axial measurements with table feed perpendicular to the image plane, and 3D techniques in which the readout direction is oriented parallel to the direction of the table feed. The present invention relates primarily to the first group.
The optimal implementation of a method in the group of 2-D axial measurements with table feed for pillar to the image plane differs depending on the sequence type. Two basic families within this method can be differentiated again. Sequences with short TR such as TrueFISP or turboflash belong to a first family. With these sequences it is possible to acquire the data of a single slice continuously (successively) in the center of the magnet while the patient (or in general the examination subject) is driven with constant velocity
                              v          table                =                  d                                    N              PE                        ⁢            TR                                              (                  1          ⁢          a                )                                          v          table                =                  d                                    N              r                        ⁢            TR                                              (                  1          ⁢          b                )            through the measurement volume of the MR system, wherein TR designates the time between two successive excitation slices, and NPE designates the number of phase coding steps per slice in a Cartesian acquisition technique (1a) or, respectively, Nr designates the number of radial spokes per slice in a radial acquisition technique (1b), and d designates the interval between adjacent slices.
With this successive acquisition technique (excitation of the slices in succession one after another), the data of a first slice are completely acquired before the data acquisition of an additional slice is begun.
Sequences with longer TR—such as T1-weighted flash techniques or turbo spin echo sequences—in which a TR from 100 ms to multiple seconds is required (for example in order to achieve a desired contrast or, respectively, in order to maintain a signal to be measured) belong to a second family. In this sequence family, the successive methods just described would be inefficient since the long TR leads to very low table velocities, and therefore to very long MR measurements given a slice interval d of approximately 3 mm to approximately 8 mm according to Formula (1a/b) that is typical in MRT.
Therefore, in this second sequence family the individual slices are acquired with an interleaved acquisition technique (as is frequently also the case in conventional MR measurements given a stationary examination bed) in which data of other slices are acquired between the successive excitation of a slice. In the case of a continuously moved examination bed, between two excitations of the same slice the location of the excitation is also adapted to the velocity of the movement of the examination bed such that the same slice in the examination subject is respectively encountered, assuming that the examination subject is not displaced with respect to the bed.
For example, an examination region (thus the volume within the patient from which MR images should be acquired) is subdivided into multiple slices stacks, each composed of Ns slices. The slice stacks are measured successively. During the measurement of a slice stack, the measurement position follows a fixed anatomical position within the examination subject traveling with the examination bed. The velocity of the examination bed is selected such that the travel distance during the acquisition time of a slice stack is equal to the extent of that slice stack:
                              v          table                =                                            N              s                        ⁢            d                                              N              PE                        ⁢            TR                                              (                  2          ⁢          a                )                                          v          table                =                                                            N                s                            ⁢              d                                                      N                r                            ⁢              TR                                .                                    (                  2          ⁢          b                )            
The velocity of the examination bed (and therefore the efficiency) with interleaved MR acquisition is thus increased by a factor Ns relative to the successive MR technique. For example, since the B0 field of every real MR system is not ideally homogenous and the gradient fields are not ideally linear, the different measurement positions can lead to smearing or ghost artifacts depending on the acquisition technique that is used.
The successive measurement of slice stacks that are subdivided into Ns slices that was just described has the disadvantage that different slices of a slice stack are measured in different ways, which leads to different distortions of MR images reconstructed from the acquired data (which MR images assume different positions within the slice stack) due to the mentioned imperfections of the MR system. After the assembly of the MR images, these lead to discontinuities (in particular at the stack boundaries) since anatomically adjacent slices that were associated with different slice stacks assume oppositional positions within their respective slice stack. This problem is addressed by a technique known as the “Sliding Multislice” (SMS) technique that is described in the article by Fautz and Kannengieβer, “Sliding Multislice (SMS): A New Technique for Minimum FOV Usage in Axial Continuously Moving-Table Acquisitions”, Magnetic Resonance in Medicine 55:363-370 (2006). In the special interleaved SMS acquisition technique, identical k-space lines of different slices are respectively measured at the same location within the MR system. All slices are therefore measured similarly again.
Among other things, a high homogeneity of the B0 field is necessary for a uniform fat suppression in the acquired MR images.
With an appropriate design of the magnet of the MR system and stationary shim measures (for example ferromagnetic materials mounted at a suitable location at the MR system), a specified homogeneity of the magnetic field is achieved in the defined spherical or cylindrical measurement volume of the MR system. The homogeneity of the magnetic field in the measurement volume that is achieved in such a manner is, however, disrupted by the introduction of a patient or other examination subject into that volume.
In a shim adjustment, the actual field curve of the magnetic field can be measured and linear gradient fields can be determined that are superimposed on the basic magnetic field in order to at least approximately reestablish the homogeneity of the magnetic field in a target volume (for example the measurement volume or a portion of the measurement volume from which diagnostic measurement data will be acquired during a subsequent MR examination). The linear gradient fields are normally generated with the use of the three gradient coils that are also used for the spatial coding. For this purpose, for each gradient coil a current, known as the offset current, is superimposed on the gradient current (for example for the spatial coding) that is predetermined by the pulse sequence. The linear field contributions that are realized with such a gradient offset current are also called shim channels of a first order.
The homogeneity in the target volume can normally be increased by additional special shim coils having a suitable power supply and amplifiers. Five such shim coils typically exist in MR systems, which shim coils produce a field contribution with a spatial dependency described in Cartesian coordinates by xy, xz, yz, x2-y2, z2. This field contribution increases linearly with the current through the respective coil. In the literature, such a contribution is also referred to as being produced by a shim channel, and such additional shim coils are thus also called shim channels of a second order.
However, both the examination subject in the measurement volume and the shim currents in the shim channels influence the effective field strength, and therefore the resonance frequency of the MR system. Therefore, after the examination subject has been moved into the measurement volume and after the shim currents have been defined and set, the RF center frequency is re-determined in a procedure known as a frequency adjustment. This is particularly necessary when spectral saturation or excitation methods are used during the data acquisition (for example for fat suppression).
If data from different target volumes in the patient are acquired during the MR examination, the shim adjustment and the subsequent frequency adjustment that are described above must be repeated for each target volume. This is the case if the examination subject is moved between individual measurements in the examination.
MR examination techniques are already available in which the patient or an examination subject on an examination bed is driven continuously through the MR system together with the examination bed during the acquisition of the measurement data (referred to herein the measurement). For example, such MR examination techniques are distributed by Siemens AG under the product name “syngo TimCT Oncology”.
In these particular situations when the measurement itself takes place given a continuously moved patient bed (also known as “move during scan” (MDS), “continuously moving table MRI” (CMT) or “syngo TimCT”), the target volume constantly changes.
In numerous publications about such MR measurements that are implemented during continuous movement of the examination subject (the patient, for example), patient-specific adjustment measurements (for example to determine the necessary shim currents and the RF center frequencies) are completely omitted, and instead patient-independent system values or empirically determined experimental values for the load-dependent adjustment values are used, even though limitations in the image quality must be accepted as a consequence.
One exception is the work by A. Shankaranarayanan and J. Brittain: “Continuous Adjustment of Calibration Values for Improved Image Quality in Continuously Moving Table Imaging”, Proc. Intl. Soc. Mag. Reson. Med. 11 (2004), #103. There it is proposed to determine adjustment values at 16 stations distributed over the entire body of a patient before the actual MR measurement in a “prescan”. During the continuous travel of the examination bed in the actual radial MR measurement, the determined adjustment values (shim currents of 1st order, RF center frequency and reference transmitter voltage) are respectively adapted after the acquisition of Nr spokes. Since the actual bed position normally does not coincide with one of the 16 node points, the current adjustment values are determined with linear interpolation.
From U.S. Pat. No. 7,145,338, a method is known in which adjustment values are determined during continuous travel of the bed. In a subsequent high-resolution diagnostic examination, these adjustment values are then applied depending on the position of the patient bed.
A shim adjustment in which the gradient offset currents are determined depending on the position of the examination bed can also be implemented given continuous travel. The gradient offset currents determined during the shim adjustment are then applied both in subsequent further adjustment measurements (in particular RF center frequency adjustment) and in subsequent (for example high-resolution) diagnostic MR examinations, depending on the current bed position and therefore on the current position of the examination subject in the measurement volume.
The current through the shim channels of the second order is normally not varied given MR measurements with continuous travel of the examination subject. The reason is that these channels, at least in current MR systems, are not similarly equipped with powerful amplifiers like the shim channels of the first order (gradient coils), and therefore too long of a time duration would have to pass before the desired B0 field contribution were set. Rapid switching during such continuous travel therefore is not possible.
Such an MR examination technique with continuous travel of the examination subject is used in a procedure known as “tumor staging”, among other things. In oncology, what is designated as tumor staging is the portion of the diagnosis that serves to establish the degree of propagation and the metastisizing of a malignant tumor. The MR examination normally uses a spoiled gradient echo sequence (also known as a FLASH sequence, “Fast Low Angle SHot”) with spectral fat suppression.
In spectral fat suppression, use is made of the fact that the resonance frequency of protons that are bound in fat molecules differs from the resonance frequency of those that are bound in water molecules by 3.3 to 3.5 parts per million (ppm), thus by approximately 217 Hz at 1.5 T. Therefore, in spectral fat suppression a spectrally selective excitation pulse that flips the protons that are bound in fat molecules in the transverse plane, and that does not affect protons that are bound in water molecules, is switched (activated) before each acquisition module, or for a series of acquisition modules. The magnetic resonance signals emitted by the fat-bound protons in such a manner is known as the fat signal, and subsequently dephased with a spoiler gradient. In signals acquired immediately following this spoiler gradient by means of the following acquisition modules in the sequence, the fat signal then no longer produces a contribution, or at the least has at most a strongly suppressed signal contribution. An “acquisition module” is a portion of the overall sequence that is executed for selective excitation of a slice, the coding of the signal emitted that results from this excitation, and the readout of this signal. Multiple acquisition modules of each slice are normally necessary for complete data acquisition of the respective slice. In spectral fat suppression, a constant B0 field (and therefore a respective constant resonance frequency for each of water and fat) within the slice is a requirement for a uniform fat suppression in the images that are calculated (reconstructed) from the data acquired in such a manner. Due to the aforementioned individual disruption of the B0 field by the patient, this is achieved only when a shim and frequency adjustment is implemented as described above.
However, in practice massive ghost and smearing artifacts are often observed (primarily in the throat region of an examined patient) with the setting of the adjustment values as described above depending on the current bed position in examinations with continuously moving examination subject, which artifacts hinder a diagnosis on the basis of the MR images acquired during continuous travel, or make it entirely impossible. These artifacts are not observed, or at least not to this degree, given a corresponding conventional Flash measurement (given a stationary patient bed) and in other body regions, even given a traveling bed. The throat region is particularly important for tumor staging since cancer can propagate (metastasize) from its origin to lymph nodes of the neck or the cervical spinal region.